"If a locked container alarms during screening and is not marked as containing a declared firearm, TSA will cut the lock in order to resolve the alarm."

Whoa.

I thought the rule was that there MUST be nothing placed on the outside of the suitcase to indicate that there is a firearm inside. Has that changed?

I know what the TSA advisory says, but the TSA is noted for "advising" things that are contrary to law. Such as "advising" travelers to turn over the keys to the screening agent. That is explicitly prohibited in the law.